This invention relates to an optical paper detector and more particularly is directed to a phototransistor array for detecting multiple sheets of paper in a printing press feed system. It is particularly constructed for use by the Bureau of Engraving and Printing for detecting the presence of more than one sheet of paper in a printing press feed system during the printing of paper money.
In the past, the presence of more than one sheet of paper currency was sensed by a small lens system positioned near the paper feed mechanism of the printing machines. A small spot was customarily sensed on the area of the paper which forms the border of two adjacent paper bills. This area of the paper is not printed during either the printing operation or the overprinting operation whereby seals and serial numbers are added. This area is also where the cuts are made to form individual bills from a sheet of bills.
The smallness of the sensing area used in conventional systems can create problems since even a slight variation in paper density or paper alignment in the sensing area will effect the reading produced. Variations in paper density alone can vary be as much as 40% the amount of light striking the optical paper detector. Misalignment of the paper can result in printed material being inadvertently placed within the sensing area; this is particularly true during overprinting where the seals and serial numbers are printed on partially printed bills. Such misalignment can drastically affect the amount of light from the source reaching the optical paper detector. Variations in the density of the paper or variations in the paper alignment or a combination of these two types of variations can cause false triggering thereby resulting in an unnecessary and undesireable feed interruption.